(a) Description of Prior Art
Growth hormone (GH) or somatotropin, secreted by the pituitary gland constitute a family of hormones which biological activity is fundamental for the linear growth of a young organism but also for the maintenance of the integrity at its adult state. GH acts directly or indirectly on the peripheral organs by stimulating the synthesis of growth factors (insulin-like growth factor-I or IGF-I) or of their receptors (epidermal growth factor or EGF). The direct action of GH is of the type referred to as anti-insulinic, which favors the lipolysis at the level of adipose tissues. Through its action on IGF-I (somatomedin C) synthesis and secretion, GH stimulates the growth of the cartilage and the bones (structural growth), the protein synthesis and the cellular proliferation in multiple peripheral organs, including muscles and the skin. Through its biological activity, GH participates within adults at the maintenance of a protein anabolism state, and plays a primary role in the tissue regeneration phenomenon after a trauma.
The decrease of GH secretion with the age, demonstrated in humans and animals, favors a metabolic shift towards catabolism which initiates or participates to the ageing of an organism. The loss in muscle mass, the accumulation of adipose tissues, the bone demineralization, the loss of tissue regeneration capacity after an injury, which are observed in elderly, correlate with the decrease in the secretion of GH.
GH is thus a physiological anabolic agent absolutely necessary for the linear growth of children and which controls the protein metabolism in adults.
Growth hormone (GH) secretion is regulated by two hypothalamic peptides: GH-releasing hormone (GHRH), which exerts stimulatory effect on GH release and somatostatin which exhibits an inhibitory influence. In the last few years, several investigators have demonstrated that GH secretion can also be stimulated by synthetic oligopeptides termed GH-releasing peptides (GHRP) such as hexarelin and various hexarelin analogs (Ghigo et al., European Journal of Endocrinology, 136, 445-460, 1997). These compounds act through a mechanism which is distinct from that of GHRH (C. Y. Bowers, in “Xenobiotic Growth Hormone Secretagogues”, Eds. B. Bercu and R. F. Walker, Pg. 9-28, Springer-Verlag, N.Y. 1996) and by interaction with specific receptors localized in the hypothalamus and pituitary gland ((a) G. Muccioli et al., Journal of Endocrinology, 157, 99-106, 1998; (b) G. Muccioli, “Tissue Distribution of GHRP Receptors in Humans”, Abstracts IV European Congress of Endocrinology, Sevilla, Spain, 1998). Recently it was demonstrated that GHRP receptors are present not only in the hypothalamo-pituitary system but even in various human tissues not generally associated with GH release (G. Muccioli et al., see above (a)).
GHRPs and their antagonists are described, for example, in the following publications: C. Y. Bowers, supra, R. Deghenghi, “Growth Hormone Releasing Peptides”, ibidem, 1996, pg. 85-102; R. Deghenghi et al., “Small Peptides as Potent Releasers of Growth Hormone”, J. Ped. End. Metab., 8, pg. 311-313, 1996; R. Deghenghi, “The Development of Impervious Peptides as Growth Hormone Secretagogues”, Acta Paediatr. Suppl., 423, pg. 85-87, 1997; K. Veeraraganavan et al., “Growth Hormone Releasing Peptides (GHRP) Binding to Porcine Anterior Pituitary and Hypothalamic Membranes”, Life Sci., 50, Pg. 1149-1155, 1992; and T. C. Somers et al., “Low Molecular Weight Peptidomimetic Growth Hormone Secretagogues, WO 96/15148 (May 23, 1996).
The human GH has been produced by genetic engineering for about ten years. Until recently most of the uses of GH were concerned with growth delay in children and now the uses of GH in adults are studied. The pharmacological uses of GH, GHRPs and growth hormone secretagogues and may be classified in the following three major categories.
(b) Children Growth
Treatments with recombinant human growth hormone have been shown to stimulate growth in children with pituitary dwarfism, renal insufficiencies, Turner's syndrome and short stature. Recombinant human GH is presently commercialized in Europe and in the United States for children's growth retardation caused by a GH deficiency and for children's renal insufficiencies. The other uses are under clinical trial investigation.
(c) Long Term Treatment for Adults and Elderly Patients
A decrease in GH secretion causes changes in body composition during aging. Preliminary studies of one-year treatment with recombinant human GH reported an increase in the muscle mass and in the thickness of skin, a decrease in fat mass with a slight increase in bone density in a population of aged patients. With respect to osteoporosis, recent studies suggest that recombinant human GH does not increase bone mineralization but it is suggested that it may prevent bone demineralization in post-menopausal women. Further studies are currently underway to demonstrate this theory.
(d) Short Term Treatment in Adults and Elderly Patients
In preclinical and clinical studies, growth hormone has been shown to stimulate protein anabolism and healing in cases of bum, AIDS and cancer, in wound and bone healing.
GH, GHRPs and growth hormone secretagogues are also intended for veterinary pharmacological uses. GH, GHRPs and growth hormone secretagogues stimulate growth in pigs during its fattening period by favoring the deposition of muscle tissues instead of adipose tissues and increase milk production in cows, and this without any undesired side effects which would endanger the health of the animals and without any residue in the meat or milk being produced. The bovine somatotropin (BST) is presently commercialized in the United States.
Most of the clinical studies presently undertaken were conducted with recombinant GH. The GHRPs and growth hormone secretagogues are considered as a second generation product destined to replace in the near future the uses of GH in most instances. Accordingly, the use of GHRPs and growth hormone secretagogues present a number of advantages over the use of GH per se.
Therefore, there is a need for compounds which, when administered to a mammal, act as growth hormone secretagogues.